NoNonsense Forum is a free, open source, PHP-based simple discussion forum. It favours removing barriers to conversation rather than massaging egos.
There is no database; all data is just RSS. Starting a new thread just creates a new RSS feed. Replying just adds an item to the feed.
There is no registration screen and e-mail confirmation to go through just to post; NoNonsense Forum asks you for a name and password when you post, but this name and password forms a unique token, protecting the name from other people using it. Every time you want to post, just use the same name and password to keep the same name. It couldn’t be simpler!